The Mechanism Of Biochar Stimulate Denitrification And The Application Of Immobilized Biochar In Denitrifying Biofilter At Low Temperature

With the rapid development of industrialization and modern agriculture,a large amount of industrial wastewater and agricultural effluent are discharged into ground and surface water bodies,causing extremely serious nitrogen pollution.It is known that nitrogen,especially nitrate and nitrite,can not only cause many environmental problems,but also a threat to human health.In nature,denitrification is the main way to remove nitrate and nitrite.But this process is difficult to accomplish without proper conditions,such as temperature and pH.Therefore,it is important and meaningful to find a technique that could promote denitrification.And how to strengthen denitrification of wastewater has always been a hot topic in the field of wastewater treatment in China.This study can be divided into two stages according to the different research objects.In the first stage,the effects of biochars pyrolyzed at different temperatures(300?C,500?C and 800?C)on denitrification were investigated.The results showed that biochar had significant effects on the denitrification capacity of microorganisms.Such effects were related to the pyrolysis temperature of biochar.Specially,biochar pyrolyzed at 300?C could promote denitrification while biochar pyrolyzed at 800?C presented a conspicuously inhibiting effect on denitrification of microorganisms.The results of denitrification showed that the nitrate and total nitrogen removal efficiency in reactors exposed to biochar at 300?C were 94% and 82%,which is remarkably higher than that of control(27% and 15%).In the meanwhile,the accumulation of nitrous oxide in the reactors with biochar at 300?C were only 22% of that in control.In order to further understand the mechanism of biochar affecting denitrification,this study examined the molecular biological indicators of microorganisms in each reactor.The results evidenced that electron transport system activity(ETSA)and denitrifying enzymes(NAR,NIR,NOR and NOS)activities of microorganisms were all sensitive to biochars.Biochar at 300?C increased the activity of ETSA,NAR,NIR and NOS by 36.9%,28.0%,116% and 235%,respectively.And the ETS and NAR activities of reactors exposed to biochar at 800?C were significantly inhibited.That is the reason why the denitrification in these reactors cannot proceed smoothly.In addition,high-throughput sequencing of microorganisms also confirmed that biochar at 300?C can significantly change the microbial community structure and increase the relative abundance of denitrifiers such as Pseudomonas,thereby enhancing denitrification process.The second stage of this study is based on exploring effects of immobilized biochar on denitrifying biofilter at low temperature.In this experiment,the influent and effluent in each denitrifying biofilter were monitored regularly.The results showed that the nitrate removal efficiency in reactor containing immobilized biochar was 98% and there is no nitrite in the effluent.But in control,nitrate removal efficiency is only 62% and the concentration of nitrite accumulated to 9.9 mg/L.In other words,as a filler,immobilized biochar can effectively stimulate denitrification under low temperature conditions.According to the test results of lactate dehydrogenase(LDH)activity,we can conclude that the microbial cell membranes were intact in reactor exposed to biochar.Compared to the control,the microorganism showed no significant decline in these reactors.Moreover,the denitrifying enzymes activities were decreased at low temperature.But immobilized biochar could exist in the denitrifying biofilter for a long time,increasing the activity of NAR,NIR,NOS and reducing the emission of nitrous oxide.This is important for environmental protection and control of greenhouse gases.In summary,biochar can significantly stimulate denitrification.The reason of this phenomenon is mainly because biochar could increase the ETSA,denitrifying enzymes activities and change community structures of microorganisms.After the biochars were prepared into small balls by immobilization,they can be applied to the denitrifying biofilter and improve the efficiency and stability of denitrifying biofilter at low temperature.